This invention relates to improvements in a lock-up control system for an automatic transmission including a torque converter provided with a lock-up clutch.
Hitherto it has been well known that an automatic transmission provided with a lock-up clutch is mounted on an automotive vehicle. In the torque converter, when a difference in rotational speed is made between a torque converter power input shaft driven by an engine and an automatic transmission power input shaft, a torque increasing action is produced under the action of a stator. Accordingly, when the engine is operated at a vehicle starting or the like, a large rotational speed difference is produced between the torque converter power input shaft and the automatic transmission power input shaft thereby providing the torque increasing action under the action of the torque converter, thus obtaining a good vehicle starting characteristics.
However, when the rotational speed difference between the torque converter power input shaft and the torque converter power output shaft becomes small, for example, during a vehicle cruising at a constant vehicle speed, no torque increasing action can be made, so that the torque converter functions merely as a fluid coupling thereby lowering a torque transmission efficiency. In this regard, it has been employed to directly couple the power input and output shafts of the torque converter upon engagement of the lock-up clutch, i.e., to directly couple the engine and the automatic transmission, thereby preventing the torque transmission efficiency from lowering and improving fuel economy.
Even with the above lock-up clutch, the following problems have been encountered: The torque increasing action by the torque converter cannot be obtained in a lock-up condition in which the lock-up clutch is engaged, and therefore a response characteristics is degraded owing to torque shortage even if an acceleration operation is made by depressing an accelerator pedal in the lock-up condition.
In view of the above, a lock-up control technique is proposed and disclosed in Japanese Patent Provisional Publication No. 59-43256. This lock-up control technique is arranged as follows in order to solve the problems in which the torque increasing action cannot be employed and therefore no sufficient acceleration can be made if the lock-up condition is released when an accelerator pedal is depressed to make a rapid acceleration during a vehicle running in the lock-up condition: A section for detecting a movement speed of an accelerator pedal is provided. Additionally, a lock-up releasing section is provided to output a signal for causing the lock-up clutch to be compulsorily disengaged when the accelerator pedal movement speed is not lower than a predetermined level. Under the action of these sections, the lock-up condition is immediately released regardless of running conditions of the vehicle when a rapid acceleration is made. This generates a rotational speed difference between the engine and the torque converter power output shaft thereby providing the torque increasing action, thus effectively using the torque converter to make a sufficient acceleration.
However, drawbacks have been encountered even in the above lock-up control technique. That is, during a low speed vehicle running (for example, at a vehicle speed of 20 to 40 km/h) upon depression of the accelerator pedal by the driver in the lock-up condition of the torque converter, if the driver further depresses the accelerator pedal intending an acceleration, the lock-up condition is immediately released. As a result, the path of torque transmission from the engine is momentarily interrupted thereby generating a large depression shock (due to sudden lowering in torque of the power output shaft of the transmission). This will be discussed in detail with reference to FIG. 8 which is a time chart of the torque characteristics of the transmission power output shaft, obtained when an accelerator pedal is further depressed to make an acceleration in a driving condition in which the accelerator pedal has been depressed by the driver under the lock-up condition of the torque converter. In the time chart, a thin line indicates the torque characteristics obtained when the lock-up condition is maintained, demonstrating the fact that the driver""s acceleration requirement cannot be met since a rising response of the transmission output shaft torque is low while the maximum level of the output shaft torque is low. The thick line indicates the torque characteristics obtained when the lock-up condition is immediately released, demonstrating the fact that the output shaft torque abruptly lowers when the lock-up condition is released, and thereafter the output shaft torque abruptly rises thereby producing the large depression shock.
It is, therefore, an object of the present invention is to provide an improved lock-up control system for an automatic transmission, which can effectively overcome drawbacks encountered in conventional lock-up systems and techniques for an automatic transmission.
Another object of the present invention is to provide an improved lock-up control system for an automatic transmission, which can effectively meet a driver""s acceleration requirement while suppressing the feeling of a depression shock due to immediately releasing a lock-up condition of a torque converter when an accelerator pedal is depressed during a vehicle running in a driving condition under a lock-up condition of the torque converter.
An aspect of the present invention resides in a lock-up control system for an automatic transmission including a torque converter having a power input element connected to an engine of a vehicle and a power output element connected to a power input shaft of the transmission, and a lock-up clutch housed in the torque converter and being capable of directly coupling the power input element and the power output element of the torque converter. The lock-up control system comprises an accelerator pedal depression amount sensor for detecting a depression amount of an accelerator pedal, and a controller. The controller includes an accelerator pedal movement speed calculating section for calculating a movement speed of the accelerator pedal. The controller includes a lock-up control section adapted to disengage the lock-up clutch in a condition in which a driving point of the vehicle is within a lock-up condition releasing region in a set lock-up schedule, and to engage the lock-up clutch in a condition in which the driving point is within a lock-up condition applying region. The lock-up control section includes a first lock-up releasing control section adapted to gradually disengage the lock-up clutch from an engaging state and put the lock-up clutch into a disengaging state when the accelerator pedal movement speed is not lower than a set speed and an initial accelerator pedal depression amount in the accelerator pedal depression amount is not smaller than a set accelerator pedal depression amount to establish a driving condition of the vehicle under the engaging state of the lock-up clutch.
Another aspect of the present invention resides in a lock-up control system for an automatic transmission including a torque converter having a power input element connected to an engine of a vehicle and a power output element connected to a power input shaft of the transmission, and a lock-up clutch housed in the torque converter and being capable of directly coupling the power input element and the power output element of the torque converter. The lock-up control system comprises means for detecting a depression amount of an accelerator pedal, means for calculating a movement speed of the accelerator pedal; and lock-up control means for disengaging the lock-up clutch in a condition in which a driving point of the vehicle is within a lock-up condition releasing region in a set lock-up schedule, and for engaging the lock-up clutch in a condition in which the driving point is within a lock-up condition applying region. The lock-up control means includes a first lock-up releasing control section adapted to gradually disengage the lock-up clutch from an engaging state and put the lock-up clutch into a disengaging state when the accelerator pedal movement speed is not lower than a set speed and an initial accelerator pedal depression amount in the accelerator pedal depression amount is not smaller than a set accelerator pedal depression amount to establish a driving condition of the vehicle under the engaging state of the lock-up clutch.
A further aspect of the present invention resides in a method of controlling lock-up for an automatic transmission including a torque converter having a power input element connected to an engine of a vehicle and a power output element connected to a power input shaft of the transmission, and a lock-up clutch housed in the torque converter and being capable of directly coupling the power input element and the power output element of the torque converter. The lock-up controlling method comprises (a) detecting a depression amount of an accelerator pedal; (b) calculating a movement speed of the accelerator pedal; and (c) disengaging the lock-up clutch in a condition in which a driving point of the vehicle is within a lock-up condition releasing region in a set lock-up schedule, and engaging the lock-up clutch in a condition in which the driving point is within a lock-up condition applying region. The disengaging and disengaging the lock-up clutch (c) includes gradually disengaging the lock-up clutch from an engaging state and put the lock-up clutch into a disengaging state when the accelerator pedal movement speed is not lower than a set speed and an initial accelerator pedal depression amount in the accelerator pedal depression amount is not smaller than a set accelerator pedal depression amount to establish a driving condition of the vehicle under the engaging state of the lock-up clutch.